This invention relates to the field of optical parametric oscillators and in particular to the control of the output spectral linewidth and line center of laser pumped optical parametric oscillators.
The optical parametric oscillator, which consists of a nonlinear gain medium inserted into a resonator cavity, provides a convenient method for generating widely tunable radiation from a single device. Recent material developments such as beta barium borate and urea have brought about new interest in optical parametric oscillators, especially in the visible region of the spectrum.
The output frequency bandwidth of an optical parametric oscillator is dependent on the spectral linewidth and angular divergence of the pump laser. This is due to the fact that the output frequencies are determined by energy and momentum conservation wherein the frequency of the pump must equal the sum of the signal and idler frequencies. Additionally, this is due to the fact that the extraordinary index of refraction in birefringement crystals is a function of the angle of incidence and that all the indices vary with wavelength.
Many applications, such as submarine laser communications, require a narrowband source, much narrower in fact, than is routinely available from a conventional optical parametric oscillator. Traditionally, line narrowing has been achieved by insertion of frequency dispersive elements into the cavity. Single mode operation has been achieved by using a grating, beam expander and an etalon within the resonator cavity. Unfortunately, however, narrowline operation is often accompanied by a decrease in efficiency and output energy due to the fact that the intracavity dispersive elements possess optical loss and their finite physical size increases the resonator length, both of which raise the oscillation threshold.